As we know all organisms are composed of cells. So it is
very important thing to study about the cell.
A cell has sub cellular units. All sub cellular units have special structure
and functions. Let’s explore that structure and functions one by one.
Structures and functions of organelles and sub cellular components.
Cell wall
• Outer envelope of plant cell.
Animal cells do not have cell walls
• Made up of cellulose and
pectin. Hemicellulose, lignin suberin may also be
present.
•
May have primary and secondary walls. Explain the chemical components of
middle lamella, primary cell wall and secondary cell wall.
• Cell wall has pits through
which cytoplasm of adjoining cells join through
plasmodesmata.
• Functions of cell wall.
• Plasma membrane
• Outer limit of protoplasm.
• Illustrate the structure of
the fluid mosaic model of the plasma membrane,
consisting of a bilayer of lipids, integral proteins and peripheral
proteins.
Movable lipids give the fluidity whereas the arrangement of protein
molecules
gives the mosaic structure.
• Other membranes in the cell
also have the same structure.
• Dynamic boundary of cell.
• Permits the entry of water,
ions and certain organic molecules.
• Regulates the exit of waste
materials.
• Maintains an osmotic balance
within the cell.
• Receives information through
receptors and transmit signals to co-ordinate
Nucleus
• Stores genetic information of
the cell.
• Most cells possess one
nucleus, some have several nuclei. (e.g., fungi)
• Explain the structure of
nucleus, consisting of double membrane envelope,
nuclear sap,
chromatin, nuclear pores, and nucleolus.
• Explain the functions of
nucleus.
• Control cellular activities.
• Synthesizes DNA to produce new
nuclei for cell divisions.
• Synthesizes ribosomes and RNA
required for protein synthesis.
Ribosomes
• Consists of a large and a small sub unit
composed of r- RNA and protein.
• Synthesizes proteins coded by
m-RNA with the help of t-RNA.
• Endoplasmic reticulum
• Network of internal membranes forming
flattened or tubular sacs
separating
cytosol from ER lumen. Continuous with the outer membrane
of nuclear
envelope
• Two types of ER; Rough ER and Smooth ER
Rough ER
• Rough ER consists of flattened sacs, and
ribosomes bound to surface
• Proteins synthesized by ribosomes move into
lumen of ER.
• Synthesize membrane phospholipids and
membrane glycolipids
• Transport enzymes and other proteins within
cell. Produce
transmission vesicles for
transport
Smooth ER
• Network of tubular sacs without ribosomes
• Presence of membrane bound enzymes
• Synthesize
lipids, steroids and carbohydrates
• Transport within cell. Produce transmission
vesicles for transport.
• Detoxification
• Stores Ca 2+ ions
Golgi complex
• Stacks of flattened vesicles
or Golgi bodies
• Inner and outer surfaces can
be identified
• Illustrate the structure of
Golgi complex
• Functions of Golgi complex
• Proteins and lipids manufactured
in endoplasmic reticulum (ER) are transported
through channels of ER or vesicles into Golgi complexes
• Lipids and proteins combine
with polysaccharides to form glycolipids and
glycoproteins repectively
within the cisternae of Golgi complex
• When necessary ,vesicles
containing glycoproteins or glycolipids are budded off
from cisternae and move into other locations of the cell
• Produce lysosomes
Lysosomes
• Membrane bounded vesicles contributing to digestive activity
• Contain hydrolytic enzymes
which catalyze breakdown of carbohydrates,
proteins, lipids and nucleic acids.
• Digest food particles received
by phagocytosis
• Digest worn out organelles
•
Transport residue material out of cell by exocytosis
• Autolysis causing cell death
Microbodies (Peroxisome & glyoxysome)
• Membrane bound vesicles with
oxidizing enzymes.
• Two common types of
microbodies
• Glyoxisomes – present in plant
cells
• Peroxysomes – present in plant
and animal cells.
• Enzymes in glyoxisome converts
fat into carbohydrate
• Enzymes in peroxysome catalyze
the break-down of H 2O2
• Function of peroxysome
• Detoxification of peroxides
• Photorespiration in plants
Mitochondria
• Organelle bound by two
membranes. Inner membrane bearing cristae.
• Illustrate the ultra structure
of mitochondria with two membranes, cristae,
matrix with ribosomes and DNA.
• Explain the function of
mitochondria
• Matrix carries enzymes
for reactions of Krebs cycle. Cristae
carry out
electron transport chain and oxidative phosphorylation system.
• Synthesize ATP by oxidizing
reduced coenzymes using oxygen.
Chloroplasts
• Double membrane bound
organelle found only in plant cells
• Explain the gross structure of
chloroplast using an electron micrograph
including outer membrane, inner membrane, thylakoids, grana and stroma
with
ribosomes DNA and starch granules.
• Thylakoids contain
photosynthetic pigments.
• Function of chloroplast -
photosynthesis
Cytoskeleton
• Supporting structure of
cytoplasm made of microtubules and protein filaments.
Dynamic structure, breaking and reforming as needed.
• Explain structure of three
types of components.
• Microtubules, Actin
filaments, Intermediate filaments
• Functions of cytoskeleton
• Gives strength to cytoplasm
• Support organelle of cell
• Movement of cytoplasm,
cytoplasmic streaming, moves organelle and
chromosomes when necessa
Cillia and Flagella
• Made of microtubules, with a
9+2 structure. Covered by plasma membrane.
Bound to a basal body.
• Illustrate the structure with
micrographs.
• Functions of flagella and
cilia includes locomotion and transport of material on
cell surface.
Centriole
• A pair of cylindrical
structures made of microtubules found in animal cells.
Produce aster and spindle in cell divisions.
Vacuoles
• A large structure, bound by
tonoplast, filled with liquid found in plant cells.
• Stores water and other
materials such as sugars, ions and pigments
• Maintains water balance of
cell
• Gives turgidity and strength
to cell.
• Produce colours in some plants
with sap pigments
• Stores soluble substances needed for cellular
activities.
Cell junctions
• Structures at which cytoplasm
of adjoining cells are joined.
• Plasmodesmata in plant cells.
• Three types in animal cells
• Tight junctions – connect the
plasma membranes of adjacent cells tightly
e.g., in
epithelial cells of gut preventing leakages through intercellular
space.
• Anchor junctions –
mechanically attach the cytoskeletons of adjoining
cells for strong binding eg. skin
epithelium
• Gap junctions (Communication
junctions) – allow signal and material
exchange between adjacent cells through direct connections. e.g., heart
muscles
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